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But it is important to remember how many VP picks have helped the ticket, he says

Zelizer: Romney could use a boost, since he's 7 points behind Obama in the latest CNN poll

Mitt Romney has taken many people by surprise by announcing that his vice presidential running mate will be Wisconsin Representative Paul Ryan. The decision excites many conservatives who have been calling on Romney to go big. They believe Ryan will inject some juice into a campaign they feel has been lackluster and put the focus on the policy differences between Romney and President Obama.

The primary risk with Ryan, from what we currently know about him, is that his controversial budget plan and tough line on Medicare could energize liberals and alienate elderly voters in key states like Florida. He also lacks foreign policy expertise and has spent most of his career in the city that conservatives hate, Washington. In recent decades, the record of vice presidential running mates who have come right out of the House is not very good.

The risk of making the wrong choice for vice president was highlighted in 2008 by Sen. John McCain's selection of Alaska Gov. Sarah Palin, a decision many believe helped torpedo the Republican nominee. Palin struggled before the media, appearing inexperienced and unprepared for the Oval Office. She also used aggressive conservative rhetoric that undercut McCain's appeal to independents.

Julian Zelizer

But for all the talk about what can go wrong with vice presidential nominations, it is important to remember how many of these picks have helped the ticket. Very often vice presidential running mates have proven to be excellent on the campaign trail. They have compensated for the weaknesses of the person at the top of the ticket, they have played the role of attack dog and they have complemented the presidential candidate's strengths.

In 1952, Gen. Dwight Eisenhower was determined to remain above the partisan fray. He wanted a campaign that highlighted his military record as America was bogged down in a military stalemate in Korea. California Sen. Richard Nixon served as an effective attack dog, taking on Democrat Adlai Stevenson for being too weak in fighting communism, branding him "Adlai the Appeaser" and saying the kinds of things Eisenhower wanted to avoid saying himself.

Although a scandal involving a contribution fund Nixon maintained as senator almost caused him to be dumped from the ticket, Nixon turned the table on Democrats by going on television and delivering the famous "Checkers Speech," in which he turned public sympathy in his favor. After hearing the speech, Eisenhower met with Nixon and said, "Dick, you're my boy."

Photos:Romney and Ryan set as Republican ticket

Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Republican presidential candidate Mitt Romney, left, jokes with Wisconsin Rep. Paul Ryan Saturday, August 11, after announcing him as his running mate at a campaign event on the USS Wisconsin in Norfolk, Virginia.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Romney, left, and Ryan wave at supporters in front of the USS Wisconsin.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Romney supporters wait for him to introduce Ryan as his vice presidential running mate.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Romney and Ryan wave to supporters.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Romney supporters sing the U.S. national anthem as they wait for Romney to introduce Ryan as his vice presidential running mate.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Romney and Ryan shake hands as they embark on a four-day, four-state bus tour through Virginia, North Carolina, Florida and Ohio.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Ryan speaks after Romney introduces him as his running mate.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Ryan speaks after Romney introduces him as his vice-presidential pick.

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Photos:Romney and Ryan set as Republican ticket

Romney and Ryan set as Republican ticket – Ryan addresses a crowd of supporters in Norfolk, Virginia.

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A few years later, Nixon fell victim to a good vice presidential pick when he ran for president against John F. Kennedy in 1960. Though he was disliked by many liberals, former Senate Majority Leader Lyndon Johnson gave the Democrats strong appeal in Southern states and also helped bring an impressive level of Washington expertise to the ticket.

In 1968, Nixon picked Maryland Gov. Spiro Agnew, who certainly brought his fair share of problems. Agnew was mocked by his opponents as a little known politician and uttered his share of gaffes as well as ethnic slurs. Yet Agnew did contribute to Nixon's victory by simultaneously appealing to the center, given his moderate record as governor in most policy areas, and to Southerners, given his increasingly hard line on racial issues such as school busing and law and order. Like Nixon in 1952, Agnew served as the attack dog and allowed Nixon, who had remade himself from an avid Cold Warrior into a foreign policy expert, to maintain his new image.

In 1976, Minnesota Sen. Walter Mondale appealed to traditional Democrats such as organized labor and African-Americans who were distrustful of Jimmy Carter, an unknown Southerner who did not seem sympathetic to the party's core ideas. When Carter ran against Ronald Reagan in 1980, George H.W. Bush gave the GOP ticket immense foreign policy expertise and Washington experience that helped Reagan push back against criticism that he was just a lightweight Hollywood actor who would be unable to function in the corridors of Washington.

Tennessee Sen. Al Gore proved to be a pitch-perfect running mate for Bill Clinton in 1992. Clinton wanted to convey the impression that Democrats were no longer the old "tax and spend" party Republicans had enjoyed attacking. Clinton was a Southerner and part of a new generation of Democrats. Gore, a hawkish centrist who agreed with Clinton, complemented the party message.

Together, Clinton and Gore counteracted the ability of President Bush, and then in 1996 Sen. Bob Dole, to draw on the familiar campaign themes of the GOP. Simply the image of the two youthful men and their families generated excitement among voters.

In 2000, George W. Bush surprised many observers when he picked former Secretary of Defense Dick Cheney, who was heading the vice presidential selection process, as his running mate. Cheney didn't do much on the campaign trail, but he did provide a record of deep experience in Washington that counteracted Democratic charges that Bush was a novice who lacked any gravitas. In 2004, Cheney would eviscerate Sen. John Kerry's running mate, Sen. John Edwards, during a debate that made the Democrat look too young and too inexperienced.

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In 2008, Sen. Barack Obama, who already had the wind to his back, took something of a risk with Sen. Joe Biden, who was notorious for his off the cuff remarks and gaffes. But Biden added value to the ticket, compensating for Obama's perceived inexperience and proving willing to engage in tough partisan attacks that Obama shied away from.

It is possible that Ryan could help Mitt Romney in the coming months ---- and he could use a boost, since he's 7 points behind Obama in the latest CNN poll.

If Ryan's main role is to be the ideological messenger of the campaign or to become the main attraction, he probably will not be effective. Based on the history, and the limited assistance vice presidential candidates can provide, what Romney needs is for Ryan to strengthen his central message, namely that he has more economic expertise than President Obama, to convince right-wing as well as Rust Belt Republicans that Romney would be a good president and to be able to engage in the kinds of tougher attacks that conservatives are clamoring for, the kinds of attacks Vice President Biden has proven quite adept at handling for Democrats.